fix offset math in ColorTransform.concat

[Geokureli]

in this thread Thomas outlined the expected offset formula:

concat_offset = multiplier2*offset1 + offset2;

Joshua replied that it was implemented, however the formula used was:

concat_offset = multiplier1*offset2 + offset1;

the doc states

the effect is the same as applying the second color transformation after the original color transformation.

but the previous code has this backwards

Came across this when I was implementing flixel tools for adobe animate's tint and brightness color transforms, I noticed that concatenating a full brightness or full tint on top of another transform did not completely override the initial transforms values.

[Geokureli]

I'm not very well versed in unit testing visual utilities like this. For test_concat1, I clearly just took the new values and put then as the expected value, I feel like this is not great practice, and I'm open to suggestions

Edit: I see, so the test's purpose is to ensure that openfl behaves identical to air/flash, and it appears that my changes do not, perhaps concat's doc is incorrect?

Is my interpretation of the doc, wrong?